I enjoyed the callback to his dad, Robert Baratheon, who was world famous for kicking a** with a hammer... but it still doesn't make sense beyond that. Hammers shouldn't really be that effective against the walkers, even if it they were made of dragon glass, and why would he have trained to get good at fighting with a hammer when he is a blacksmith? Or at least good enough to trust himself going to fight walkers and coming out alive. The person who taught him everything he knows was able to create Valyrian steel weapons and one of the best blacksmiths in the world. I assume that is the reason for his reappearance, although they haven't shown how good of a blacksmith Gendry actually is at this pont. So why they are sending him past the wall when he is so important is beyond me.